When imaging ionizing radiation with either film or digital detection means, phosphor screens are used to transduce the ionizing radiation to visible light. Phosphor screens have the inherent problem of contributing an artifact to the image called screen mottle. Screen mottle is the combined effect of macroscopic structural mottle and microscopic grain mottle, often lumped into the term “mottle”. Generally, screen mottle contributes to the noise in phosphorescence detection, specifically high spatial frequency noise that is spatially fixed with respect to the detection means. It would be desirable to reduce or eliminate the contribution of mottle to noise in phosphorescence detection in an ionizing radiation imaging system, such as a radiographic or autoradiographic imaging system. This problem is particularly relevant to thin phosphor screens, as required for high spatial resolution radiography and autoradiography of small mammals, insects, fish, seeds, biopsy specimens, blots, gels, and the like, due to the small number of phosphor grains through a pixel equivalent column depth. Furthermore, this problem is particularly relevant in cases where a reduction in the dose of ionizing radiation to achieve a desired signal-to-noise ratio is desired, because a decrease in noise can compensate against a decrease in signal (due to reduction in dose) to maintain a desired signal-to-noise ratio.
A number of attempts to reduce or eliminate mottle have been reported in the literature. For example, reference may be made to Cleare et al, The Am. J. of Roent. And Rad. Physics, Vol. 88, No. 1, pp. 168-174 (July 1962); U.S. Published Patent Application 2006/0210135; and U.S. Pat. Nos. 1,609,703; 3,717,764; 3,936,644; 4,028,550; 4,088,894; 4,107,070; 4,208,470; 4,394,737; 4,710,637; 4,829,188; 4,891,527; 5,069,982; 5,663,005; 5,830,629; and 6,278,765. While some have achieved a measure of improvement, these attempted solutions have required added complexity and cost due to either the addition of complex materials, processes, or construction techniques, or the use of additional screens or layers. For example, some proposed solutions use a plurality of radiographic films or a plurality of phosphor screen layers. Some minimize the effective conversion efficiency of the screen or the screen speed or require the use of additional materials such as the embedding of metal strips. Others require adding extra materials such as brightening agents, combining (Ba,Sr)F, (Cl,Br): Eu+2 phosphors with particular rare earth oxyhalide phosphors, admixing a small amount of particular trivalent antimony compounds with the phosphor prior to screen preparation, or heating phosphor material while exposed to an oxygen-containing atmosphere. Still others require stabilization or a correction image data set.
U.S. Patent Application Publication No. 2007/0217713 and NewScientist.com news service, Dec. 21, 2007, describe a technique for creating higher resolution images by combining a plurality of lower resolution images. Forensic scientist and astronomers are currently applying the technique to security and astronomical images respectively, to produce higher resolution images. The method and software first acquire a series of lower resolution images while moving the subject and holding the capture device fixed or moving the capture device while holding the subject fixed. Then like pixels of the lower resolution images are combined to create a higher resolution image. Researchers are also applying this technique to radiography to obtain usable radiographic images that require less radiation dose. Radiographic images obtained in this fashion, however, are also subject to phosphor screen mottle, because the phosphor screen remains fixed in relationship to either the image capture device (whereby the mottle adds noise that is spatially fixed with respect to the plurality of images) or the subject (whereby the mottle adds noise that is spatially fixed with respect to the subject).